This invention relates to circuits for controlling a generator, motor, and brake operating in a control system and more particularly to such a control system for use in controlling an elevator car in an elevator hoistway.
In the past, attempts to control power supplied to driving components for an elevator car in a hoistway utilizing duty cycle switching of an AC power signal have been hampered by asymmetry in the power signal, and by difficulty in obtaining a stable reference signal for the triggering necessary to obtain precision in duty cycle regulation. Trigger pulses necessary to obtain long firing angles for low power operation were difficult to obtain with acceptable firing angle accuracy due to reference signal drift. Unfortunately, it is at the long firing angle that greatest trigger signal accuracy is required, since this condition corresponds to the slowest elevator car speeds, which may occur with the elevator doors open. Erratic elevator car operation with passengers embarking or disembarking is clearly undesirable. Moreover, since the usual AC power signal wave is asymmetrical, high speed operation depending upon triggering the postive pulses for one direction of elevator car movement and triggering the negative AC signal pulses for the opposite elevator car movement, have often resulted in unequal speeds of movement in opposite elevator car directions. Prior elevator car drive systems have displayed abrupt increasing and decreasing acceleration characteristics, which may cause some degree of passenger discomfort depending on the running speeds desired to be attained by the elevator car.
There is a need therefore for a control system which responds to speed and direction inputs for driving an elevator car, and which provides equal speeds in the up and down directions, linear increasing and decreasing acceleration of the car, and precise triggering at desired firing angles to obtain close control of elevator car movement over the entire elevator car speed range.